Thermal imaging is a process in which images are recorded by the use of imagewise modulated thermal energy. In general there are two types of thermal recording processes, one in which the image is generated by thermally activated transfer of a light absorbing material, the other generates the light absorbing species by thermally activated chemical or physical modification of components of the imaging medium. A review of thermal imaging methods is found in "Imaging Systems" by K. I. Jacobson R. E. Jacobson--Focal Press 1976.
Thermal energy can be delivered in a number of ways, for example by direct thermal contact or by absorption of electromagnetic radiation. Examples of radiant energy include infra-red lasers. Modulation of thermal energy can be by intensity or duration or both. For example a thermal print head comprising microscopic resistor elements is fed pulses of electrical energy which are converted into heat by the Joule effect. In a particularly useful embodiment the pulses are of fixed voltage and duration and the thermal energy delivered is then controlled by the number of such pulses sent. Radiant energy can be modulated directly by means of the energy source e.g. the voltage applied to a solid state laser.
Direct imaging by chemical change in the imaging medium usually involves an irreversible chemical reaction which takes place very rapidly at elevated temperatures--say above 100.degree. C.--but at room temperature the rate is orders of magnitude slower such that effectively the material is stable.
A particularly useful direct thermal imaging element uses an organic silver salt in combination with a reducing agent. Such systems are often referred to as `dry silver`. In this system the chemical change induced by the application of thermal energy is the reduction of the transparent silver salt to a metallic silver image.